Long-term variations of the solar wind proton parameters

Abstract

Interplanetary plasma data taken near 1 AU by a variety of spacecraft (Imp 1, Vela 2‐4, Explorer 33‐35, and Heos 1) from 1963 to 1971 are used to study the long‐term variations of the solar wind proton properties. An intercalibration among the different experiments is performed in order to obtain a coherent set of data. The most interesting result is a ∼40% reduction of the proton density between the minimum and the maximum of the solar activity cycle. The observed density variation occurs throughout the velocity range; however, a limited sample of data could suggest that at very low velocities (V < 300 km/s) the density is independent of the solar activity. The proton bulk velocity is remarkably constant during the period considered: small increases of the average speed occurring in 1968 and 1971 cannot be attributed to the solar cycle. No clear trend is exhibited by the proton temperature: differences among the various experiments probably have an instrumental origin. The density modulation has been interpreted in terms of the heliographic latitude effect proposed by Hundhausen et al. (1971). It turns out that with increasing solar activity this effect disappears, producing the observed decrease of the average density near the solar maximum.

Cosmic ray perpendicular gradient during 1962-1972

“…Solar wind observations from 1964 on provide evidence for a heliographic latitude dependence of the plasma flow properties, at least within 4-7 ø of the hellolatitude excursion of the earth and its satellites [Hundhausen et al, 1971;Smith and Rhodes, 1974;Rhodes and Smith, 1975;Diodato et al, 1974]. The daily cosmic ray intensity data detected by the Deep River neutron monitors have been used for the period 1962-1972.…”

Solar radio burst and in situ determination of interplanetary electron density

“…There is a general anticorrelation between sunspot ,number and interplanetary density, as has been pointed out by previous authors (Diadato et al, 1974;Schwenn, 1983). From Fig.…”

Long‐term solar wind electron variations between 1971 and 1978

“…Activity on the surface of the sun varies with an approximately I l-year period. Variations in several characteristics of the interplanetary magnetic field and solar wind flow near the earth have been found to be associated with this cycle [Neugebauer, 1975;Diodato et al, 1975;Hedgecock, 1975;Barne et al, 1976;Gosling et al, 1971Gosling et al, , 1976Siscoe et al, 1977;Bridge, 1976;Feldman et al, 1978a;King, 1976King, , 1979. The purpose of the present study is to extend past work on solar cycle variations to include solar wind electron thermal characteristics.…”